A fuel cell is an electric power generating system which oxidizes a fuel by an electrochemical process to directly convert the energy emitted due to an oxidation reaction into electrical energy, and which has a stack structure constituted of a plurality of layered membrane electrode assemblies formed by sandwiching both side surfaces of an electrolyte membrane for selectively transporting hydrogen ions by a pair of electrodes composed of a porous material. In particular, a solid polyelectrolyte fuel cell using a solid polymer membrane as an electrolyte is expected to find application as a vehicle-mounted electric power source because of its low cost, ease of size reduction, and high output density.
For this type of fuel cells, it is generally known that the range of 70 to 80° C. is an optimum temperature range for electric power generation. In an environment of a cold region or the like, there are cases where a prolonged time is required for the optimum temperature range to be reached after a startup, so that various types of warm-up systems are being studied. For example, Japanese Patent Application Laid-Open No. 2002-313388 discloses a technique for warming up a fuel cell while a vehicle is traveling by controlling the amount of self-heating of the fuel cell by carrying out a low-efficiency operation in which electric power generation efficiency is lower than that of a normal operation. According to the technique, an output voltage of the fuel cell is set to have a voltage value that is lower than a voltage value based on its I-V characteristic (current-to-voltage characteristic) to carry out a warm-up operation from self-heating by increasing a heat loss of the fuel cell. This obviates the need for mounting a warm-up apparatus, providing enhanced convenience.    [Patent Document 1] Japanese Patent Application Laid-Open No. 2002-313388